1. Field of the Invention
The invention disclosed herein relates to performing measurements of a property of a formation from a borehole.
2. Description of the Related Art
Exploration for hydrocarbons generally requires performing several types of measurements of properties of a geologic formation. Many types of measurements are conducted in a borehole that penetrates the formation. Measuring a property of the formation from the borehole is called “well logging” in the art of hydrocarbon exploration.
One type of measurement used in hydrocarbon exploration is induction well logging. Induction well logging is based on the principle of inducing current loops in the formation and measuring a signal resulting from the current loops. In a simplified version of an instrument for performing induction well logging, an alternating current is conducted through a transmitter coil, thereby, creating an alternating magnetic field in the formation. From the alternating magnetic field, loops of alternating current are created in the formation. The alternating current loops in the formation, in turn, produce their own alternating magnetic field. When the alternating magnetic field produced by the alternating current loops in the formation intersect a receiver coil in the instrument, a current in induced in the receiver coil. The current induced in the receiver coil (referred to as an output signal) is proportional to the conductivity of the formation, which may include contributions from different regions.
Eccentricity of the instrument in the borehole may cause an error in the output signal due to electric current flowing in the direction of the axis of the borehole. The current may be arbitrarily distributed over the cross-section of the borehole. In addition, a part of this current may flow in a conductive support in the instrument and may contribute to the error in the output signal. The effect of the current in the conductive support may be reduced by making the support in separate electrically disconnected pieces. However, the electrically disconnected pieces may not completely solve this problem.
Another approach to the problem of eccentricity of the induction instrument in the borehole is disclosed in U.S. Pat. No. 6,933,726. This approach requires positioning a transversal receiver coil symmetrically with respect to the support. Use of this approach, however, requires high positioning accuracy.
Therefore, what are needed are techniques that do not require high positioning accuracy of a transversal receiver coil in a logging instrument. Preferably, the techniques allow the transversal receiver coil to be placed asymmetrical to a current flowing internal to the logging instrument without significant degradation of an output signal from the receiver coil.